1. Field of the Invention
This invention relates to a substation (for the distribution of electrical energy) protected against arcing faults, i.e. accidental arcs.
2. Background Information
Arcing faults tend to occur, e.g. in low voltage substations, when a direct electrical connection is created between the conductors or between a conductor and ground, without the presence of a dead short circuit, e.g. if the insulation is penetrated or as a result of incorrect operation. The live parts at different potentials are thereby bridged by a plasma column.
On account of the very high temperature of the plasma, the immediate vicinity of the arc is severely heated, and consequently the pressure in the substation increases so rapidly that it reaches its maximum in as little as 15 to 20 ms (milliseconds). The pressure stress is frequently greater than the mechanical strength of the doors and partitions of the switchgear cabinets. These parts then explode and their parts blow out at high velocities.
If people are in the vicinity, they can be severely burned by the hot gases and hot parts. The radiation caused by the plasma column can also cause electro-ophthalmia, i.e. the burning of the eyes caused by the UV radiation generated by an electrical arc.
Swiss Patent No. 676 174 discloses a device to detect arcing faults, and this device uses an optical fiber conductor sensor to detect an arc in a substation. The arc detector thereby extends through several switchgear cells. This device transmits an error message regardless of the point of origin of the arcing fault.
One disadvantage of the arrangement disclosed in the aforementioned Swiss patent is that in the event of an arcing fault, it is not possible to perform selective disconnects from the supply, e.g. by continuing to supply power to consumers by means of reserve incoming supplies. Further, it is not disclosed how arcing faults which do not last longer than a specified time, e.g. 5 ms, can be extinguished so that a selective short circuit protection is not adversely affected.
The publication "Elektrotechnik 1982", Volume 6, pp. 23-27, in particular its FIG. 5, and Hungarian Patent Application No. 169 992 disclose a high-speed grounding switch which, it is claimed, will extinguish an arcing fault in less than 5 ms. FIG. 6 of the "Electrotechnik" publication illustrates a power network with a reserve supply, in which the use of the high-speed grounding switch is described.
In the event of an arcing fault, e.g. in the primary feed, the high-speed grounding switch is actuated in every case. Of course, people in the area are reliably protected, but it cannot be considered acceptable that, in areas where the power supply must be very reliable, entire areas served would have to be without power for rather long periods of time, e.g. on account of false tripping caused by interference from light from external sources or arcing faults upstream of the incoming feeder circuit breaker.
Heretofore, the field of the invention includes numerous guidelines, patent literature, articles in the technical press and miscellaneous publications on the topic of "protection against arcing faults". However, in spite of numerous proposed solutions, the number of accidents has increased significantly since the 1960s.